A low-molecular organic electroluminescent material has been known as a material for forming such a known organic electroluminescence device.
Known examples of the low-molecular organic electroluminescent material are light emitting materials such as a chelate complex (e.g. a tris(8-quinolinol)aluminum complex), a coumarin complex, a tetraphenyl butadiene derivative, a bisstyrylarylene derivative and an oxadiazole derivative, all of which are reported to emit light of blue to red in visible region. Thus, such a material is expected to be applied to a color display device.
Vacuum deposition is employed for forming thin films using the low-molecular electroluminescence organic material, where the low-molecular electroluminescence organic material is sublimated with good thermal stability to vapor-deposit the thin films on a substrate, thereby providing a high-performance organic electroluminescence device (for instance, Patent Document 1 and the like).
However, the deposition method requires high-vacuum facilities and complicated manufacturing processes. In addition, it has been difficult to partition the film(s) into each color section of red, green and blue by using the deposition method. Another problem of the deposition method is that a material-use efficiency is low.
As another method of forming an organic electroluminescent material into films, a coating method has been known.
According to the coating method, which is generally used for forming films from a polymer organic electroluminescent material, an organic electroluminescent material dissolved in a solvent is used for forming thin films of the organic electroluminescent material (for instance, Patent Document 2 and the like). Exemplary advantages of the coating method are that thin films can be favorably formed from the organic electroluminescent material in a simplified manner with low cost, and that partition of the film(s) into color sections can be performed in a facilitated manner.
However, synthetic pathway of the polymer organic electroluminescent material is complex, and high purification of the polymer organic electroluminescent material is difficult. Moreover, properties such as luminous efficiency, lifetime, chromatic purity of the polymer organic electroluminescent material are inferior to those of the low-molecular organic electroluminescent material. Particularly, a blue-emitting polymer organic electroluminescent material having a high luminous efficiency, a long lifetime and a high chromatic purity has not been known in a practical application.
In view of the above, it has been proposed to form films of the low-molecular organic electroluminescent material by coating method.
However, a coating composition with the low-molecular organic electroluminescent material dissolved therein has been problematic in terms of its solubility, viscosity and the like.
In forming thin films of the organic electroluminescent material by coating method, the organic electroluminescent material needs to be dissolved in a solvent.
When the polymer organic electroluminescent material is employed, a coating composition with the polymer organic electroluminescent material dissolved in a solvent such as toluene, xylene or tetralin is typically known (for instance, Patent Documents 3, 4, 5 and the like).
However, a low-molecular organic electroluminescent material, which is an insoluble material, is not favorably dissolved in such a solvent as described above in forming films from the low-molecular organic electroluminescent material.
While the coating method is only applicable to materials whose solubility is more than a predetermined value (e.g. 0.5 mass % or more), the low-molecular organic electroluminescent material generally exhibits solubility of 0.1 mass % to 0.2 mass %. Accordingly, such a low solubility of the low-molecular material has prevented the low-molecular organic electroluminescent material from being formed into a film by coating method.
Although the coating method has been recently found applicable to forming films of the low-molecular materials (for instance, Patent Document 6 and the like), the solubility of the low-molecular material according to Patent Document 6 is still insufficient. Moreover, in the invention disclosed in Patent Document 6, an organic electroluminescent material to be used is limited to a predetermined compound that is soluble in a solvent such as toluene. Accordingly, an insoluble compound serving as a high-performance organic electroluminescent material cannot be used. Consequently, when an organic electroluminescence device is actually manufactured by using the coating composition disclosed in Patent Document 6, the obtained organic electroluminescence device does not exhibit sufficient performance (luminous efficiency, lifetime, chromatic purity and the like).
Patent Document 1: WO2004/018587
Patent Document 2: JP-A-2003-229256
Patent Document 3: WO2005/059267
Patent Document 4: JP-A-2002-313561
Patent Document 5: JP-A-2004-119351
Patent Document 6: JP-A-2006-190759